This application is based on Japanese Patent Application No. 2001-196884 filed on Jun. 28, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a projection optical system, and more particularly to a projection optical system suitable for use in a rear projection apparatus for projecting an enlarged image of an image displayed on, for example, a light valve (such as a liquid crystal panel) onto a screen.
2. Description of the Prior Art
Conventionally, there are known rear projection apparatus that project an image displayed on a small-size light valve onto a screen by the use of a projection optical system, and there has been increasing demand for more light-weight and compact models of such projection apparatus. However, making a projection apparatus light-weight makes it impossible to greatly increase the number of lens elements used in it, and making a projection apparatus compact requires giving its projection optical system a wider angle of view, which makes it difficult to satisfactorily correct various aberrations (in particular, chromatic aberration and distortion). For these reasons, such a projection optical system needs to have an appropriately designed optical construction. Moreover, variations in the temperature inside the apparatus caused by illumination light degrade optical performance, and therefore the focal lengths of the individual lens elements and their arrangement need to be determined with consideration given to the temperature properties of lens materials.
By using a retrofocus-type projection optical system having a wide angle of view and a long back focal length, and turning its optical path midway, it is possible to reduce the thickness of a rear projection apparatus in its depth direction. As projection optical systems having an optical path turned midway, various types have been proposed that are composed of a front lens unit and a rear lens unit. Here, the front lens unit starts with, from the conjugate enlargement side, a first and a second lens element both negative, includes an aspherical surface, and has a negative optical power as a whole; the rear lens unit includes an aspherical surface, and has a positive optical power as a whole. Examples of projection optical systems constructed in this way are disclosed in U.S Pat. No. 5,973,848, Japanese Patent Application Laid-Open No. 2001-42211, and U.S Pat. No. 6,144,503.
However, the conventional examples mentioned above have the following disadvantages. An attempt to give them a wider angle of view makes it impossible to satisfactorily correct chromatic aberration with as few lens elements as possible. Variations in temperature cause great variations in their back focal length. In particular, in the projection optical systems disclosed in U.S Pat. No. 5,973,848 and Japanese Patent Application Laid-Open No. 2001-42211, making the angle of view wider makes it impossible to secure sufficient space to permit the optical path to be turned at an acute angle. Thus, these constructions: are unsuitable to reduce the thickness of a rear projection apparatus in the depth direction or reduce the protrusion below the screen. On the other hand, the projection optical systems disclosed in U.S Pat. No. 6,144,503 is not telecentric at the conjugate reduction Side. Thus, using a prism or the like that separates or integrates optical paths in it leads to low contrast, uneven colors, uneven brightness, and the like. This makes it difficult to obtain satisfactory images.
An object of the present invention is to provide a light-weight, compact projection optical system that offers a wide angle of view and high performance and that produces high-resolution images.
To achieve the above object, according to one aspect of the present invention, a projection optical system that, by using projection light emanating from a light valve illuminated by illumination light, projects an enlarged image of an image displayed on the light valve onto a screen is provided with, from the conjugate enlargement side: a front lens unit having a negative optical power as a whole, including at least one aspherical surface, having a first lens element having a negative optical power disposed at the conjugate-enlargement-side end thereof, and having a second lens element having a negative optical power disposed on the conjugate reduction side of the first lens element; optical path turning means, including a reflecting surface, for turning the light path of the light emanating from the front lens unit; a rear lens unit having a positive optical power as a whole, including at least one aspherical surface, and including a third lens element made of an anomalous dispersion material and having a positive optical power; and projection light preparing means disposed between the rear lens unit and the light valve and having at least one of the function of separating the illumination light and the projection light and the function of integrating together light of different colors. Here, the center of the light valve is not located on the optical axis of the rear lens unit. Moreover, the following conditions are fulfilled:
xe2x80x8348 less than xcfx89 less than 60xe2x80x83xe2x80x83(1)
3 less than D1/f less than 7xe2x80x83xe2x80x83(2)
Vd greater than 75xe2x80x83xe2x80x83(3)
0.52 less than (Ngxe2x88x92Nf)/(Nfxe2x88x92Nc) less than 0.56xe2x80x83xe2x80x83(4)
where
According to another aspect of the present invention, a projection optical system that, by using projection light emanating from a light valve illuminated by illumination light, projects an enlarged image of an image displayed on the light valve onto a screen, is provided with, from a conjugate enlargement side: a front lens unit having a negative optical power as a whole, including at least one aspherical surface, having a first lens element having a negative optical power disposed at the conjugate-enlargement-side end thereof and having a second lens element having a negative optical power disposed on the conjugate reduction side of the first lens element, optical path turning means, including a reflecting surface, for turning the light path of the light emanating from the front lens unit; a rear lens unit having a positive optical power as a whole and including at least one aspherical surface; and projection light preparing means disposed between the rear lens unit and the light valve and having at least one of the function of separating the illumination light and the projection light and the function of integrating together light of different colors. Here, the center of the light valve is not located on the optical axis of the rear lens unit. Moreover, the following conditions are fulfilled:
48 less than xcfx89 less than 60xe2x80x83xe2x80x83(1)
3 less than D1/f less than 7xe2x80x83xe2x80x83(2)
xe2x88x920.5 less than xe2x88x92ffa/fra less than 0.5xe2x80x83xe2x80x83(5)
where
According to still another aspect of the present invention, a rear projection apparatus is provided with: a light valve for displaying an image; a screen onto which the image displayed on the light valve is projected from behind; and a projection optical system that projects an enlarged, image of the image displayed on the light valve onto the screen. Here, the projection optical system is constructed as described above.